Multiple copying process with improved image retention

ABSTRACT

The disclosed process produces a plurality of copies of an original by repeating development of the same electrostatic latent image of the original once formed on a charge retentive member and transfer of the thus developed image onto plural sheets of recording paper, wherein the development is effected by using a magnetic brush having a non-magnetic sleeve with an electrically-insulating surface layer and a two-component developer containing toner and a carrier with a volume resistivity of less than 10 10  Ω·cm and higher than 10 7  Ω·cm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrophotographic copying process forproducing a plurality of copies from the same electrostatic latent imageof an original formed on an electrostatic charge retentive member, byrepeating development of the electrostatic latent image and transfer ofthe thus developed image to image receiving members.

2. Description of the Prior Art

To obtain a plurality of high quality copies from the same electrostaticlatent image formed on an electrostatic charge retentive member, it isimportant to maintain the stable electrostatic latent image of theoriginal on the electrostatic charge retentive member, such as a rotaryphotosensitive drum, throughout the copy production cycle. In practice,however, the electrostatic charge leaks from the electrostatic latentimage for various reasons, so that it is very difficult to ensure highpicture quality throughout a plurality of copies made by the repeateddevelopment and transfer of the one latent image. One of the causes ofsuch deterioration of the electrostatic latent image is the transfer ofelectrostatic charge during the development. Such electrostatic chargetransfer is due to leakage of the electrostatic charge of the latentimage from the electrostatic charge retentive member through thedeveloper or injection of unwanted electrostatic charge onto theelectrostatic charge retentive member through the developer. Morespecifically, it has been known that such deterioration of theelectrostatic latent image tends to occur in the case of a magneticbrush developing unit which uses a combination of two-componentdeveloper containing magnetic carrier and toner and anelectrically-conductive non-magnetic sleeve such as an aluminium sleeve.It is believed that the magnetic carrier particles are electricallyconductive and allow the leakage and injection of electrostatic charge.

Several proposals have been made to overcome the shortcoming. Forinstance, one proposal is to insulate the surfaces of the magneticcarrier particles by coating the surfaces with resin or the like. Thisproposal is effective but has shortcomings in that the process ofproducing the magnetic carrier is complicated by the need for specialinsulating treatment. Further the process is expensive because of theneed for special equipment to accomplish the insulating treatment.

Another proposal is to provide an electric insulating layer on thesurface of the non-magnetic sleeve. This proposal can be effectedcomparatively easily at relatively low cost but has a shortcoming inthat, in the case of copying a solid picture, front edges of the copiedpicture tend to lose picture density as the number of copies increases.Despite that, high picture quality can be maintained from the first copyto the last copy in the case of copying a linear picture, as confirmedby experiments.

The decrease in print density at the front edge of the solid picture maybe mitigated by increasing the developing efficiency, e.g. by increasingthe toner concentration. However, in such a case the linear picturemight be over-developed and might become thick. Further, the backgroundof the image might be undesirously developed to some extent. In thisconnection it should be noted that the single document image may includeboth the solid picture and the linear picture. Therefore, in the knownprocess it is very difficult to develop both the linear picture and thesolid picture in an optimum manner.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to obviate theaforesaid shortcomings of the prior art by providing an improvedmultiple copying process, which process prevents deterioration of anelectrostatic latent image throughout production of a plurality ofcopies in an easy and inexpensive manner so as to facilitate stablereproduction of both linear pictures and solid pictures even in the samedocument image over a large number of copies.

To fulfill the object, the process of the present invention comprisesthe steps of producing an electrostatic latent image of an original tobe copied on an electrostatic charge retentive member, developing thelatent image by a magnetic brush developing unit using a combination ofa non-magnetic sleeve with an electrically-insulating surface layer anda two-component developer containing a low-resistance carrier with avolume resistivity of less than 10¹¹ Ω·cm, preferably less than 10¹⁰Ω·cm and toner, transferring the thus developed image onto an imagereceiving member, and repeating said developing and said transferringsteps until the desired number of copies are formed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is made to theaccompanying drawings, in which:

FIG. 1 is a schematic sectional view of a multiple copying machine forcarrying out the multiple copying process of the present invention;

FIG. 2 is a cross sectional view showing a non-magnetic sleeve with aninsulating layer; and

FIG. 3 is a graph showing variations of electric potentials ofelectrostatic latent images when using an electrically-conductive sleeveand an electrically-insulating sleeve.

In FIGS. 1 and 2, 1 is an original, 2 and 3 are pairs of feedingrollers, 4 is an illuminating lamp, 5 is an optical system, 6 is arotary photosensitive drum as an electrostatic charge retentive member,7 is a discharging lamp, 8 is a corona charger, 9 is a toner developingunit of the magnetic brush development type, 10 is a toner imagetransfer station, 11 is a recording paper cassette, 12 is recordingpaper as an image receiving member, 13 is a pickup roller, 14 isregister rollers, 15 is a transfer roller, 16 is a guide, 17 is a pairof guide rollers, 18 is a fixing unit, 19 is a pair of feed rollers, 20is a cleaner brush, 21 is a fan, 22 is a filter, 23 is a housing, 24 isa shaft, 25 is a separator pawl, 26 is an air duct, 27 is an opening, 28and 29 are switches, 30 is a non-magnetic sleeve, 30A is an insulatinglayer, and 31 is a permanent magnet.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically shows the construction of a multiple copyingmachine to which the multiple copying process of the present inventionis applied. When a sheet-like original 1 to be copied is placed on aninclined original table and moved in the direction of the arrow A, twopairs of feeding rollers 2 and 3 act to forward the original 1. Whenmoving between the feeding rollers 2 and 3, the original 1 passes abovean illuminating lamp 4 and an optical system 5. The optical system 5projects an image of the illuminated original 1 onto a rotaryphotosensitive drum 6 acting as an electrostatic charge retentivemember. The photosensitive drum 6 rotates in the direction of the arrowB, and after being discharged by a discharging lamp 7, thephotosensitive drum 6 is uniformly charged by a corona charger 8, sothat upon projection of the aforesaid image of the original 1, anelectrostatic latent image is formed thereon. This latent image istoner-developed by a toner developing unit 9, and then the thusdeveloped image is forwarded to a toner image transfer station 10 as thephotosensitive drum 6 rotates.

A recording paper cassette 11 carries a number of sheets of recordingpaper 12 each sheet acting as an image receiving member. A swayingrotary pickup roller 13 picks up the recording paper 12 one sheet at atime, so as to feed the recording paper 12 to the toner image transferstation 10 through the pair of register rollers 14. In the toner imagetransfer station 10, the recording paper 12 passes between thephotosensitive drum 6 and a transfer roller 15, so that the recordingpaper 12 comes in contact with the aforesaid toner image and causes thetoner image to be transferred to the recording paper 12. During thetransfer, the recording paper 12 is kept in tight contact with the tonerimage and hence with the photosensitive drum 6, and after beingseparated from the drum 6, as will be described later, the recordingpaper 12 is forwarded along a guide 16 by the pair of rollers 17 towarda fixing unit 18. After the toner image is fixed at the fixing unit 18,the recording paper 12 is discharged to the outside of the copyingmachine by the pair of feed rollers 19.

When one copy is to be formed from the latent image on thephotosensitive drum 6, the toner image on the photosensitive drum 6 isnot wholly transferred to the recording paper 12, but partially remainson the drum 6, so that a rotary cleaner brush 20 brushes off theresidual toner from the photosensitive drum 6. A fan 21 generates an airflow to suck the thus brushed off toner, and a filter 22 collects thetoner particles from the air flow. A housing 23 encloses the cleanerbrush 20 and the fan 21 to produce an effective suction for sucking thetoner and to prevent the toner particles from being scattered in theapparatus.

On the other hand, when multiple copies are to be made from oneelectrostatic latent image once formed on the drum 6 by repeating thetoner development and the transfer of the developed images to aplurality of recording papers 12, it is preferable not to clean thetoner image after each transfer. Thus, the embodiment of FIG. 1 allowsthe cleaner brush 20 to be moved away from the drum 6 by pivoting thecleaner brush about a shaft 24.

To separate the recording paper 12 from the drum 6, the embodiment ofFIG. 1 uses a combination of a separator pawl 25 and air flow theretofrom the opening 27 of an air duct 26 defined by the housing 23.Switches 28 and 29 detect the position of the original 1, so as tofacilitate the start of the copying machine upon insertion of theoriginal 1 thereto and the control of the operations of various partsthereof in synchronism with the movement of the original 1.

In the present invention, the toner developing unit 9 is of the magneticbrush development type, and a two-component developer containingmagnetic carrier and toner is used in the developing unit 9. Anon-magnetic sleeve 30 such as an aluminium sleeve is rotatably mountedin the developing unit 9 and a permanent magnet assembly 31 is disposedwithin the non-magnetic sleeve 30.

To prevent the deterioration of the electrostatic latent image on thedrum 6 during the repeated developments, according to the presentinvention an electrically insulating layer 30A is applied on the surfaceof the non-magnetic sleeve 30 of the developing unit 9 of the magneticbrush development type as illustrated in FIG. 2. The insulating layer30A effectively prevents the leakage of electrostatic charge via thedeveloper and the sleeve 30 and accordingly prevents the reduction ofthe potential of the electrostatic latent image.

Tests were made on the variation of the potential of an electrostaticimage as the number of copies thereof increased in the case of using acombination of an electrically-conductive aluminium sleeve and carriertreated by regular surface oxidation so as to have a resistivity of notless than 10¹⁰ Ω·cm and the case of using a combination of an aluminiumsleeve with an aluminium oxide layer formed on the surface thereof andsaid carrier. FIG. 3 shows the result of the tests. Curve A of thefigure shows the variation in the case of using theelectrically-conductive sleeve. The electrostatic potential of theelectrostatic image decreased greatly as the number of copies increased.On the other hand, in the case of using the electrically-insulatingsleeve, the potential of the electrostatic latent image substantiallyunchanged even when the number of copies increased, as shown by curve B.

The tests of making a plurality of copies from one electrostatic latentimage showed that, in the case of linear pictures, the last one of thethus made copies maintained substantially the same picture quality asthat of the first copy, whereas in the case of solid pictures, the printdensity at front edges of the copied image became lower as the number ofcopies increased. To prevent the deterioration of the picture quality inthe case of making a plurality of copies from one electrostatic latentimage, further tests were made by using different developers containingcarriers with different resistivities. As a result, it was found throughthe tests that, contrary to the conventional common sense, if anelectrically-insulating sleeve was used, the smaller the resistivity ofthe carrier was in a certain range, the smaller the deterioration of thepicture quality was after making a plurality of copies from oneelectrostatic latent image. In short, it was confirmed by various teststhat the suitable range of volume resistivity of the carrier was 10⁷ to10¹¹ Ω·cm, preferably 10⁸ to 10¹⁰ Ω·cm.

More specifically, five kinds of carriers C₁ through C₅ were prepared asshown in Table 1.

                  TABLE 1                                                         ______________________________________                                                                 Volume resistivity                                   Carrier Treatment        (Ω · cm)                              ______________________________________                                        C.sub.1 Iron powder without                                                                            2.0 × 10.sup.4                                         surface oxidation treatment                                           C.sub.2 Iron powder with light                                                                         6.5 × 10.sup.6                                         surface oxidation treatment                                           C.sub.3 Iron powder with medium                                                                        1.3 × 10.sup.9                                         surface oxidation treatment                                           C.sub.4 Iron powder with heavy                                                                         .sup. 1.4 × 10.sup.11                                  surface oxidation treatment                                           C.sub.5 Iron powder coated                                                                             .sup. 4.7 × 10.sup.13                                  with acrylic resin                                                    ______________________________________                                    

All the carriers C₁ through C₅ had substantially the same grain shapes,and the grain size distribution of the carriers were adjusted so that90% by weight of each carrier was in a grain size range of 50 to 100μ.The volume resistivities of Table 1 were determined by placing eachcarrier in a space between two cylindrical piston-like electrodesdisposed with a spacing of 15 mm therebetween, the electrodes having aneffective surface area of 5 cm², loading a weight of 1 kg from the upperside electrode, applying a DC voltage of 100 V across the electrodes,and taking measurement one minute after applying the DC voltage.

Two-component developers were prepared by mixing toner and each of thecarriers C₁ through C₅, so that each of the developers had a tonerconcentration of 8% by weight. Tests of picture quality were carried outby using the developers thus prepared in two kinds of developing units,namely one developing unit having the electrically-conductive aluminiumsleeve and another developing unit having the aluminium sleeve withaluminium oxide layer. The picture quality was evaluated by taking 20copies from one electrostatic latent image and checking the picturequality of the twentieth copy. Table 2 shows the result of the picturequality tests.

                                      TABLE 2                                     __________________________________________________________________________    Electrically-insulating sleeve                                                                     Electrically-conductive sleeve                           Carrier                                                                           Solid picture                                                                         Linear picture                                                                         Solid picture                                                                         Linear picture                                   __________________________________________________________________________    C.sub.1                                                                           Fair    Unsatisfactory                                                                         Unsatisfactory                                                                        Unsatisfactory                                   C.sub.2                                                                           Good    Fair     Unsatisfactory                                                                        Unsatisfactory                                   C.sub.3                                                                           Good    Good     Fair    Unsatisfactory                                   C.sub.4                                                                           Unsatisfactory                                                                        Good     Fair    Fair                                             C.sub.5                                                                           Unsatisfactory                                                                        Fair     Fair    Fair                                             __________________________________________________________________________

As can be seen from Table 2, the combination of theelectrically-insulating sleeve and the low-resistance carrier providedthe same high picture quality in the twentieth copy as that of the firstcopy for both the solid picture and the linear picture being copied.Further tests were made by changing the resistivities of the carriers,and it was found that the suitable range of the volume resistivity ofthe carrier was 10⁷ to 10¹¹ Ω·cm, preferably 10⁸ to 10¹⁰ Ω·cm. Thepreferable resistivity of the electrically-insulating surface layer 30Aon the surface of the non-magnetic sleeve 30 was found to be 10⁸ to 10¹²Ω·cm, preferably about 10¹⁰ Ω·cm.

The reason why the excellent stability of the picture quality, farexceeding that expected can be achieved by using the aforesaidcombination of the low-resistance carrier and theelectrically-insulating sleeve is not clear yet. It is believed thatfactors other than the leakage of electrostatic charge contribute to thehigh picture quality achieved by the present invention, because, whendevelopers containing various electrically-insulating carriers were usedto make a plurality of copies from one electrostatic latent image, theelectric potential of the electrostatic latent image at twentieth copieswas substantially the same as that at the first copies.

It should be noted that the present invention is not restricted to theillustrated embodiment alone, and numerous modifications are possible.For instance, the non-magnetic aluminium sleeve with the aluminium oxidesurface layer used in the embodiment can be replaced by a plastic sleeveof hollow cylindrical shape. Besides, the iron powder of the carrier inthe illustrated embodiment can be replaced with powder of nickel,ferrite, or the like.

What is claimed is:
 1. A multiple copying process, comprising the stepsof:producing an electrostatic latent image of an original to beduplicated on a electrostatic charge retentive member; providing amagnetic brush developing unit including a non-magnetic sleeve and apermanent magnet assembly disposed within the sleeve, the sleeveincluding an electrically-insulating surface layer having a resistivityof 10⁸ to 10¹² ohm cm; developing the latent image to form a toner imageby said magnetic brush developing unit using a two-component developercontaining a low-resistance carrier with a volume resistivity of lessthan 10¹⁰ ohm cm and higher than 10⁸ ohm cm and a toner; transferringthe thus developed toner image onto an image receiving member; andrepeating said developing and said transferring for the same and singleelectrostatic latent image once formed on the electrostatic chargeretentive member to form a desired number of copies.
 2. A multiplecopying process as set forth in claim 1, wherein theelectrically-insulating surface layer of the non-magnetic sleeve has aresistivity of about 10¹⁰ Ω·cm.
 3. A multiple copying process as setforth in claim 1, wherein said non-magnetic sleeve is an aluminiumsleeve and said electrically insulating surface layer is an aluminiumoxide layer.
 4. A multiple copying process as set forth in claim 1,wherein said carrier is selected from the group consisting of ironpowder, nickel powder, and ferrite powder.